To answer your first question. Maybe take a couple of minutes. It's nothing major, just a technique that helps bring some mindfulness to the fore. This is good, as mindfulness is critical to the development of concentration.
We were taught during a retreat by Ajahn Tiradhammo to go slowly from the nose, eyes, forehead, scalp, top of the head, back of the head ... all the way down the back of the body, bringing awareness to each specific area of the outer form, all the way up the front, back up to the nose.
The answer to your second question:
Q: Can we focus on the tip of the nose?
Ajahn. Chah wrote:AC: That’s fine. Whatever suits you, whatever you feel comfortable with and helps you fix your mind, focus on that.
It’s like this: in teaching meditation, if we get attached to the ideals and take the guidelines too literally, it can be difficult to understand. When doing a standard meditation, such as anapanasati, first we should make the determination that right now, we are going to do this practice, and we take anapanasati as our foundation. We turn our attention to only focusing on the breath, at three points, as it passes through the nostrils, the chest, and the abdomen. When the air enters, it first passes the nose, then through the chest, then to the end point of the abdomen. As it leaves the body, the beginning is the abdomen, the middle is the chest, and the end is the nose.
We merely note it. This is a way to start controlling the mind, tying awareness to these points at the beginning, middle, and end of the inhalations and exhalations.
Before we begin, we should sit and let the mind relax first. It’s similar to doing something like sewing on a machine. When we are learning to use the sewing machine, first we just sit in front of the machine to get familiar with it and feel comfortable. Here, we just sit and breathe. Not fixing awareness on anything, we merely take note that we are breathing. We take note of whether the breath is relaxed or not and how long or short it is. Having noticed this, then we begin focusing on the inhalation and exhalation at the three points.
We practice like this until we become skilled in it and it is going smoothly. Then the next stage is to focus awareness only on the sensation of the breath at the tip of the nose or the upper lip. At this point we aren’t concerned with whether the breath is long or short, but only focus on the sensation of entering and exiting.
- http://www.what-buddha-taught.net/Books ... _Doubt.htm
Bear in mind that it's not the nostrils that is our object of concentration, it is the breathing through the nostrals. We're picking a point of contact and just being aware of the feeling of the air passing over it.